In Australia, most pet shops that deal in exotic pets can usually provide you with 'turtle food' and I assume this would be the case in other countries as well. Unlike many reptiles (and amphibians too), turtles will take non live food which means that you don't have to have a container filled with bugs in your house constantly. Small turtles can be kept in tanks. These tanks are usually about half filled with water. It is important to give the turtle somewhere to rest on land as they will probably get tired of swimming and I imagine probably drown. Many tanks setups that I have seen have a flat surface above the water which has a ramp which leads down into the water so they can get in and out of the water easily. Many species of turtles do grow quite large and will probably need to be given an outdoor inclosure when this happens. Turtles can live for a long long long time. Quite possibly longer than you depending on the species. So keep in mind that they are a long term commitment. Like any living thing, they need and deserve to be given a certain quality of life. If you are not willing to give this kind of commitment I recommend you give the little guy back to the original owners. They are lovely little creatures tho and I encourage you keep it if you believe you are willing to commit to it. All the best with it. I Hope I have been of some assitance.

Could you at least post a picture? There are only 7 species of turtle, so it's not too hard to narrow down, but all of them are quite endangered so I wonder how on Earth you got one in the first place...

EDIT:
Whoops, just found out that US English lumps Turtles, Terrapins and Tortoises all together...apologies.

I've raised aquatic turtles before... Very stinky little creatures that's tank has to be cleaned almost daily... I was like 10 at the time and I just couldn't handle it so they were taken from me... Probably for the best though...